The antigen-antibody reaction is the basis for all immunological test methods. Certain proteins known as antibodies are produced by mammals in response to the presence of an antigen, that is a foreign substance, which can be another protein or a carbohydrate. This normal body response to a foreign substance has led to the development of a number of techniques which are used to diagnose various diseases, disorders and physiological conditions. In a general sense, one component of an antibody-antigen reaction can be defined as the immunoreactive species while the corresponding component which complexes with it is considered the receptor.
In vitro tests for the presence of a suspected protein, antigen or antibody in a biological sample are carried out by adding the immunological counterpart to the biological sample. If the suspected substance is present, the resulting antigen-antibody reaction can be demonstrated by precipitation of the antigen-antibody complex. This reaction complex is generally difficult to detect visually. For this reason, either antibodies or antigens are often bound to insoluble particles, for example polymer latex particles, so that when the complex is formed, it is readily detectable from the resulting agglutination either by observing the presence of clumping or by a detectable tracer associated with the particles. Agglutination then is characterized by the clumping of particles from a suspension of particles. Further details of known agglutination methods are provided in U.S. Pat. Nos. 4,419,453 (issued Dec. 6, 1983 to Dorman et al) and 4,459,361 (issued July 10, 1984 to Gefter).
Biological samples which are assayed for various analytes may contain materials which cause nonspecific interactions of the immunoreactive species and the corresponding receptor. These nonspecific interactions may undesirably influence the assay results by showing false positives or by providing a high background so that a true positive result is difficult to detect. In addition, the polymeric particles to which immunoreactive species or receptors are attached can interact with each other due to surface charges. Moreover, the species or receptors attached to the particles can interact with each other as well, causing unwanted agglutination and inaccurate results.
Various methods have been devised to reduce nonspecific interactions, including controlling the assay pH and adding materials to modify attached proteins. One such method is described in U.S. Pat. No. 4,591,571 (issued May 27, 1986 to Kuboyama et al). This reference describes an agglutination reagent having antibodies absorbed to carrier particles. Prior to attachment, the antibodies are chemically modified with an acylating agent. It is alleged that such treatment reduces nonspecific interactions.
When an immunoreactive species was prepared using the teaching of U.S. Pat. No. 4,591,571, that is, modifying the antibodies by succinylation before attachment to beads, no signal was obtained (see Example 6 below).